510 STUDIES IN ADVANCED PHYSIOLOGY. 



2. Tympanic Membrane. The tympanum or middle 

 ear is separated from the auditory meatus by a delicate 

 membrane called the tympanic membrane. This mem- 

 brane consists of three coats : a connective tissue membrane 

 covered on the outside with the skin of the auditory meatus 

 and on the inside with the mucous membrane of the middle 

 ear. This tympanic membrane is set somewhat diagonally 

 across the end of the meatus and its surface toward the 

 meatus is concave. This concavity is produced by the 

 circumstance that the malleus is by its handle firmly at- 

 tached at the middle of this membrane and tends to pull it 

 inward. Its concavity towards the exterior possibly serves 

 to gather up the sound waves and center them on the spot 

 where the malleus is attached. 



The tympanic membrane is affected by sound waves in 

 such a way that it is set into vibrations corresponding pre- 

 cisely with the vibrations of the entering sounds. This 

 function of the membrane is well shown in the telephone. 

 The box into which one speaks has stretched across it at its 

 base, just in front of the electric magnet a delicate metal 

 diaphragm. Against this diaphragm or tympanic membrane, 

 so to speak, the voice is thrown and sets it into vibrations 

 which correspond exactly to the vibrations of the air affect- 

 ing it. Such a telephone membrane is able to catch the 

 vibrations of a high note as well as a low note. It is, in 

 fact, able to transmit to the magnet back of it all the vibra- 

 tions which strike it. 



Such is almost exactly the case with the tympanic mem- 

 brane. This membrane has, however, one or two arrange- 

 ments by means of which it can adjust itself to sounds. 

 Attached to the malleus there is a muscle called the tensor 

 tympani, by the contraction of which the tympanic mem- 

 brane is pulled further inward, thereby made more concave 

 and of course becoming more stretched. This probably 

 helps in the perception of high sounds. It is further pos- 

 sible that this tensor tympani is brought into play when we 

 listen attentively for some time to a definite sound. One 



